Semi-insulating gallium arsenide has been explored as a candidate for use in photoconductive switching. Typically, gallium arsenide is made semi-insulating by a manufacturer using compensation mechanism which causes free electron charges in the material to become trapped or immobile. Since boat-grown gallium arsenide is inherently produced with silicon impurities (a shallow donor) throughout the crystal, deep acceptors such as carbon and chrome are frequently deliberately added to the melt in order to cause the donor electrons to become trapped at the deep acceptor levels. Copper also forms deep acceptors in gallium arsenide, however, copper compensated gallium arsenide cannot be obtained through industry (without enormous expense) because copper is considered to be a contaminant in processing systems. This is because the gallium arsenide processing industry focuses almost entirely on fast optoelectronic devices and semiconductor laser technology, and copper in gallium arsenide destroys the effects that are desired in both of these applications.
One application for copper compensated, silicon doped, gallium arsenide now in the development stages, is high power photconductive switching. Therefore, there is a need to establish processing standards for copper in gallium arsenide with respect to photoconductive switching applications to accelerate maturity and transition to industry of this switching technology.
Previous experiments have been conducted to show that low resistivity, silicon-doped, gallium arsenide can be made highly resistive by doping with known amounts of copper. Experiments allowing this unique material (GaAs:Si:Cu) to be used as a bistable photoconductive switch, known as BOSS (Bulk Optically controlled Semiconductor Switch), are disclosed in U.S. Pat. No. 4,825,061 to Schoenbach et al. The fabrication of BOSS devices, as disclosed by Schoenbach et al has involved the introduction of copper into silicon-doped gallium arsenide by thermal diffusion. Copper forms two dominant deep acceptor levels in gallium arsenide known as Cu.sub.A and Cu.sub.B. These acceptor levels trap the free electrons in the crystal at the deep copper centers. Thermal diffusion processes for introduction of copper into silicon-doped gallium arsenide has been noted by J. Blane, R. H. Bube, and H. E. MacDonald, J. Appl. Phys, 32(9), 1961; Kullendorf et al, "Copper-Related Deep Level Defects in III-IV Semiconductors", J. Appl. Phys. vol. 54, pp 3203-3312, 1983; and Hasegawa J. Appl. Phys. 45, 1944 (1974).
It has also been shown that this material can be used as a photoconductive switch which means that the electrons trapped at the deep copper levels can be excited into conduction by a laser pulse of wavelength 1.06 .mu.um, and this temporary photocurrent can be extinguished by stimulating the GaAs:Si:Cu switch with another laser pulse intensity of 1.7 .mu.um. Upon excitation by the first laser pulse (a few nanoseconds in duration) the switch current rises to a peak, and then decays with the laser pulse intensity until the current through the switch is dominated by electron current instead of the electron-hole plasma created during the laser pulse (FIG. 6). At that time, the magnitude of the current is dependent on the density of electrons that were elevated from the copper center, and the lifetime of these electrons is on the order of
During the time after the first laser pulse excites the photocurrent, the switch is said to be in the "on-state", and therefore the conductivity during this phase of the switching cycle can be called the on-state conductivity. This on-state conductivity is an important parameter in the design of the switch because it determines the efficiency of the switch in delivering power to a load. The saturation of the on-state conductivity (called .sigma..sub.ss [.OMEGA.cm].sup.-1) occurs when the laser intensity is increased such that all of the electrons trapped at the Cu.sub.B level are excited into the conduction band. Past results have shown .sigma..sub.ss to be poor, and methods to control this important switching parameter are needed and have not been addressed previously.